You can probably have a successful try at making a livery yourselves.
See the article
Howto:Edit a livery on the FlightGear wiki.
As for the paintkits the article is talking about they can be found on the
Boeing 737-800 page on the livery database and ATR-72-500/Models/Liveries/paintkit.xcf. As for the Saab 340, I do not know.
As with most creative work, start by looking at photos of aircraft with liveries you like, then make some sketches with different variants of liveries you consider and compare them with each other, and only as a last step, make the livery and put it into FlightGear. You can expect both some trial and error, tweaking and in the beginning doubts, but the process and the finished livery will grow on you and will hopefully give a sense of pride some time after you are finished.
For many people with some artistic skills, making liveries is one of the easier ways to get into contributing to FlightGear (though it still involves a bit of trial and error to get it to work).